[PATCH] Ext4 Docs: Add missing bigalloc documentation.

[Ayush Ranjan]

There was a broken link for bigalloc. The page
https://ext4.wiki.kernel.org/index.php/Bigalloc was not migrated into
the current documentation sources. This patch adds the contents of that
missing page into the section for Bigalloc itself.

Signed-off-by: Ayush Ranjan <ayushr2@xxxxxxxxxxxx>
---
Please note that I have not included changes from the "Kernel Support"
and "E2fsprogs Support" sections of the original page because the
comments there seemed outdated.

 Documentation/filesystems/ext4/bigalloc.rst | 32 ++++++++++++++-------
 1 file changed, 22 insertions(+), 10 deletions(-)

diff --git a/Documentation/filesystems/ext4/bigalloc.rst b/Documentation/filesystems/ext4/bigalloc.rst
index c6d885575..72075aa60 100644
--- a/Documentation/filesystems/ext4/bigalloc.rst
+++ b/Documentation/filesystems/ext4/bigalloc.rst
@@ -9,14 +9,26 @@ ext4 code is not prepared to handle the case where the block size
 exceeds the page size. However, for a filesystem of mostly huge files,
 it is desirable to be able to allocate disk blocks in units of multiple
 blocks to reduce both fragmentation and metadata overhead. The
-`bigalloc <Bigalloc>`__ feature provides exactly this ability. The
-administrator can set a block cluster size at mkfs time (which is stored
-in the s\_log\_cluster\_size field in the superblock); from then on, the
-block bitmaps track clusters, not individual blocks. This means that
-block groups can be several gigabytes in size (instead of just 128MiB);
-however, the minimum allocation unit becomes a cluster, not a block,
-even for directories. TaoBao had a patchset to extend the âuse units of
-clusters instead of blocksâ to the extent tree, though it is not clear
-where those patches went-- they eventually morphed into âextent tree v2â
-but that code has not landed as of May 2015.
+bigalloc feature provides exactly this ability.
+
+The bigalloc feature (EXT4_FEATURE_RO_COMPAT_BIGALLOC) changes ext4 to
+use clustered allocation, so that each bit in the ext4 block allocation
+bitmap addresses a power of two number of blocks. For example, if the
+file system is mainly going to be storing large files in the 4-32
+megabyte range, it might make sense to set a cluster size of 1 megabyte.
+This means that each bit in the block allocation bitmap now addresses
+256 4k blocks. This shrinks the total size of the block allocation
+bitmaps for a 2T file system from 64 megabytes to 256 kilobytes. It also
+means that a block group addresses 32 gigabytes instead of 128 megabytes,
+also shrinking the amount of file system overhead for metadata.
+
+The administrator can set a block cluster size at mkfs time (which is
+stored in the s\_log\_cluster\_size field in the superblock); from then
+on, the block bitmaps track clusters, not individual blocks. This means
+that block groups can be several gigabytes in size (instead of just
+128MiB); however, the minimum allocation unit becomes a cluster, not a
+block, even for directories. TaoBao had a patchset to extend the âuse
+units of clusters instead of blocksâ to the extent tree, though it is
+not clear where those patches went-- they eventually morphed into
+âextent tree v2â but that code has not landed as of May 2015.
 
-- 
2.23.0